1. Field of the Invention
This invention is related to the field of Liquid Crystals (LC) in general and to the use of dopants to alter the properties of nematic LC for dynamic scattering electro-optic displays and LC containing a mixture of cholesteric and nematic material for storage scattering displays in particular.
2. Prior Art
Nematic LC and the cholesteric-nematic LC mixtures have found widespread usage in electro-optic displays such as alphanumeric displays for calculators; watches; panel meters, etc.; matrix displays; and light valves for various applications. In those applications where dc-dynamic scattering mode (DSM) is involved, prior art LC and LC mixtures have been limited by stability and scattering characteristic problems.
In seeking to overcome the scattering characteristic problems, various dopants have been added to the LC or LC mixtures. Consequently some of the scattering characteristics were improved significantly but lifetime remained as the limiting problem.
A. I. Baise et al. disclosed charge-transfer complexes as effective dopants for lowering the threshold voltage of azoxybenzene derivatives (a nematic LC) in Applied Physics Letters, Vol. 21, No. 4, p. 142 (1974). However, there was no evidence of an improvement in the lifetime of the LC for DSM display applications or of an improvement in the response characteristics of the doped materials.
The closest prior art known by the inventor hereof is in the publication by Yoshitake Ohnishi et al. entitled "Properties of Nematic Liquid Crystals Doped with Hydroquinone and p-Benzoquinone: Long-Term Dynamic Scattering Under dc Excitations" Applied Physics Letters, Vol. 24, No. 5, p. 213 (1974). In this publication the authors teach that mixtures of charge-transfer complex compounds function as dopants. However, the compounds selected by the authors are well known as weak charge-transfer complexes. Consequently, they do not undergo complete reversible electrochemical oxidation and tend to promote poor dynamic scattering efficiencies at typical operating current levels. In addition, it was disclosed by Ohnishi et al that nematic liquid crystals doped with their complexes show significant evidence of degradation within 10,000 hours at an 18 volt dc excitation level.
There are no LC or LC mixtures containing prior art dopants which have lifetimes greater than 10,000 hours and exhibit good dynamic scattering characteristics with low threshold voltages known to the inventor hereof.